A role of the toleromerase catalytic subunit, hTERT, in preventing telomeric attrition associated with cellular senescence has been established and recent studies have shown that hTERT is controlled primarily through transcriptional mechanisms. Considerable excitement has been generated by recent findings that hTERT expression may be induced in telomerase-negative cells through changes in histone acetylation and/or DNA methylation of the hTERT promoter. Since hTERT gene expression is inactivated during embryogenesis and remains transcriptionally silent in most somatic aging cells, these findings are of intense interest not only to those concerned with the causes of aging, but also to oncologists as hTERT is activated in most human cancers. Our hypothesis is that histone deacetylation and DNA methylation work in conjunction in embryonic cellular differentiation in embryonic cellular differentiation to silence the hTERT promoter and that this mechanism is attenuated in aging cells leading to a higher probability of hTERT activation and neoplasia with aging. To test this hypothesis, we have designed studies to 1) subject differentiating embryonic cells to trichostatin A and/or 5-azacytidine, which induce histone acetylation and hypomethylation of the hTERT promoter respectively, and assess hTERT activity in these cells, 2) treat young and senescent WI-38 fibroblasts with these agents to analyze the ability of histone deacetylation and/or DNA methylation to maintain hTERT gene silencing in aging cells, and 3) assess hTERT promoter regulation in differentiating embryonic cells and aging fibroblasts transfected with luciferase reporter constructs and subjected to histone acetylation and/or DNA hypomethylation. The overall purpose of this study is to further elucidate the mechanisms of histone acetylation and DNA methylation in the control of telomerase in aging and cancer. These studies are intended to strengthen the potential for development of therapeutic control of hTERT expression during aging and in age-associated diseases such as cancer.